It is generally known in the spinning industry that, apart from the unevenness in the yarn, which may have the most varied amplitudes, periodic deviations occur which later become clearly visible in the yarn (yarn conversion table) and also in the woven fabric as a result of their position side by side. This phenomenon is generally known as the moire effect. The causes of such periodic faults in the yarn originate from the processing during which disturbances periodically occur. In a ring spinning frame, such disturbances may occur, for example, as a result of drawing rollers beating. During open end or rotor spinning, such disturbances appear particularly strikingly as a result of deposits in the rotor. Because it is known what generally causes the periodic disturbances, conclusions regarding the source of distrubance can be drawn from a yarn-unevenness diagram.
In open-end spinning, with regard to moire detection, it is known that the visible spacing of thick places appear on the yarn according to the circumference of the rotor. The difficulty in detection, however, lies in the fact that the delivery speed is variable and so the moire frequency is likewise variable.
It is true that it is possible to check a yarn production afterwards by random samples by means of a spectrograph (Uster) for whether such a moire effect has appeared or not. This often results in a major loss since the production has already taken place and, in any case, a relatively large amount of faulty yarn may have been produced depending on the frequency of the checking.
It is true that it is already known to detect the point of disturbance directly and immediately by monitoring individual spindles and so to avoid a large faulty production (DT-OS No. 2.409.882). According to the earlier proposal, this is effected so that, by measuring the yarn cross-section or yarn diameter, an electrical signal is produced which is subjected to an evaluation by means of at least one non-linear correction member.
In order to detect periodic faults in the spun yarn it is necessary, in this case, to convey the signal originating from a monitored yarn through electric filters which are adjusted to the expected repetition frequency of these faults, hereinafter termed moire frequency, and a detector has to be present at each spinning station. Since the delivery speed of the spun yarn is variable, either narrow-band filters have to be used, which are variable in their midband frequency, which leads to considerable costs, or very wide-band filters have to be used so that the moire frequency is transmitted by them even at different yarn delivery speeds. In the latter case, it is true that the costs for the circuit arrangement are reduced somewhat, nevertheless the wide-band nature of the filters has the effect that a considerable proportion of the frequencies of the normal unevenness of the spun yarn can pass through. When wide-band filters are used, therefore, a moire effect is only detected if it stands out very clearly from the normal unevenness of the yarn.